Offshore production control systems typically involve a floating production control facility. Generally speaking, a hydraulic power source is mounted on the production facility. Coupled to the hydraulic power source is a master control manifold. Multiline hoses extend from the control manifold to a control pod mounted subsea adjacent the wellhead. The control pod is used to direct movement of valves mounted adjacent the wellhead. Each wellhead valve has an actuator mounted on it. Pilot valves are frequently used between the hydraulic power supply and the actuators of wellhead valves to initiate movement of wellhead valve actuators. Commonly used in subsea hydraulic control systems are pilot valves that are three-way two-position valves.
Prior designs such as the type 80 SPM valve manufactured by Koomey, Inc. have had problems with interflow. Interflow is a leakage flow from the supply port of the pilot valve through the internals of the valve and out through the vent port. This usually occurs during shifting. The type 80 valve contained a unitary sliding cage which could selectively block off the supply port while opening the vent port of the function port. Movement of a slide off of the main seal isolating the supply port from the function port simultaneously obstructed the supply and vent ports by virtue of the sliding metal to metal fit between the cage and said supply and vent ports. Further movement of the cage resulted in a blockage of the vent port and the alignment of the supply port with the function port. This design and others similar to it were prone to some interflow especially when the cage moved slightly off the main seal between the function port and the supply port. After such initial movement, there remained only the metal to metal fit between the cage and the supply port to prevent hydraulic fluid from passing through the supply port and into the vent port.
The apparatus of the present invention presents a design to eliminate interflow by virtue of two seals isolating the supply port from the vent port. Additionally, one seal on the supply port is biased in a closed position until such time as another or shear seal can be placed in position circumscribing the function port and thus isolating the vent port from the supply port.